Method for making an optical disc having a track pitch smaller than a recording beam diameter

ABSTRACT

A recording method whereby an inorganic resist made of an incomplete oxide of a transition metal is formed as a film onto a substrate and a latent image corresponding to pits is formed onto the inorganic resist by exposure. The exposure is performed by a laser beam whose intensity has been modulated by a pulse signal whose pulse height decreases in a rear portion in a length direction of the pit, thereby forming a format of a track pitch smaller than a recording beam diameter (track pitch/exposure beam diameter=0.333 to 0.833).

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2005-230408 filed in the Japanese Patent Office on Aug.9, 2005, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a recording method which is applied tomanufacturing of a master for an optical disk, the master for theoptical disk, and an optical recording medium.

2. Description of the Related Arts

In recent years, a “Blu-ray Disc (registered trademark of SonyCorporation)” format has been proposed. The Blu-ray Disc (hereinbelow,properly referred to as a BD) format is a format of a high-densityoptical disk having a recording capacity of about 25 Gbytes for aone-side single-layer or about 50 Gbytes for a one-side double-layers.

Further, according to the BD format, in order to reduce a spot diameterof a laser beam for recording and reproduction, a wavelength of a lightsource is set to 405 nm and a numerical aperture NA of an objective lensis set to a large value of 0.85. In the BD format, since the spotdiameter can be decreased to 0.58 μm, it can be decreased to about ⅕ ofthat of a DVD (Digital Versatile Disc). Further, since the numericalaperture NA of the objective lens has been increased, an angular error(called a tilt margin) which is permitted to an inclination of an anglebetween a disk surface and an optical axis of the laser beam from 90°decreases. Therefore, a cover layer covering an information layer isthinned to 0.1 mm.

In the case of an optical disk of the rewritable BD format, an on-grooverecording is used. According to the on-groove recording, the recordingis executed only to grooves in convex portions having a rough shape bywhat are called guide grooves formed on a disc substrate along recordingtracks. Grooves in a data recording area are wobbled grooves formed by amultiplexed signal of an MSK (Minimum Shift Keying) signal and an STW(Saw Tooth Wobble) signal. The MSK system is a modulating system of thewobbles and addresses have concentratedly been embedded in a specificposition of the wobbles formed by a sine wave.

By concentratedly embedding information as mentioned above, if there isa defect in such a portion, the information is influenced. Therefore,the STW is multiplexed to the wobbles of the MSK. The STW system is asystem in which the wobble shape is set to a form of a saw-tooth waveand “0” and “1” of the address information are discriminated on thebasis of the direction of the tooth. According to the STW system, sincethe same information is continuously arranged in a wide range, there issuch a feature that even if a partial defect occurs, a possibility thatit can be restored is high. In the case of the rewritable BD format, bycombining the MSK and the STW, the address information having highsurvivability for the error can be obtained.

Since the optical disc substrate has generally been formed by aninjection molding of a resin material, the low price of the optical diskhas been realized. In the injection molding of the optical discsubstrate, in order to provide patterns of grooves, pits, and the likefor the optical disk, a stamper as a master for manufacturing theoptical recording media onto which those patterns are transferred isarranged in a cavity of an injection molding apparatus.

An outline of a manufacturing method of the stamper will now bedescribed. First, a glass master is coated with a very thin photoresist(sensitive material) by a spin coating method or the like and the discis exposed by a laser of a cutting apparatus while rotating the glassmaster. A latent image of the patterns corresponding to the grooves orpits is formed onto the photoresist film by the exposure.

After that, by dropping a developer onto the rotating glass master andexecuting a developing process, the concave/convex resist patternscorresponding to the grooves or pits of the optical disk are formed ontothe glass master.

Subsequently, a metal such as nickel or the like is precipitated ontothe glass master by a plating process, peeled off, and trimmed, so thatthe stamper is obtained. The stamper is arranged in the cavity of theinjection molding apparatus. By injecting the resin into the cavity, thedisc substrate is formed.

An exposure beam diameter d at the time of exposing the photoresist filmis expressed by the following equation (1).d=1.22×λ/NA  (1)where,

-   -   λ: wavelength of the light source (laser) for exposure which is        used    -   NA: numerical aperture of the objective lens for converging a        light flux emitted from the light source onto the sensitive        material

Table 1 (refer to FIG. 5) shows the exposure wavelength of the laser, atrack pitch, a wobble amount, and the exposure beam diameter in the BDformat.

As shown in Table 1, in the BD of the high-density optical disk, a deepultraviolet laser (wavelength: 266 nm) and a resist of a stable novolaksystem are used, thereby enabling a format of a track pitch smaller thana diameter of the recording beam to be formed. That is, even in the caseof using a stable laser of 266 nm in which a track pitch of the datarecording area is equal to 320 nm and a wavelength is short, thefollowing relation is obtained.Exposure beam diameter (about 360 nm)>track pitch (320 nm)

From the above relation, the exposure beam leaks to the adjacent track.A technique in which even if the track pitch is narrowed, by uniformingan overlap of the exposure beams, fluctuations of a pit size and a pitshape are suppressed and a jitter and crosstalks of the high-densityoptical disk are reduced has been disclosed in Patent Document 1(JP-A-2003-346390).

However, in the case of the optical disk of the rewritable BD format,there is a case where the wobbled grooves extremely approach each otherin dependence on a phase relation between the adjacent wobbled grooves,and an amount of leakage of the exposure beam to the adjacent trackchanges. In addition, since an amount of leakage of the exposure beam toa land portion further changes by the wobbles, a height of land portionchanges in the BD disc formed by the steps of the exposure, development,forming and molding of the stamper, and the like.

In the rewritable BD format, since a phase-change recording film isused, a depth of groove is so shallow to be about 23 nm and in the datarecording portion, when a change amount of the height of land portiondue to the wobbles is equal to or larger than 1.4 nm, an influence ofthe height change amount increases. Thus, a fluctuation amount of apush-pull signal is large, secondary distortions of the MSK and the STWwobbles are large, and it is difficult to satisfy the standard.

A technique which can solve the problem in the case of using the organicresist in the related art and enables the high-density optical disk tobe manufactured has been disclosed in Patent Document 2(JP-A-2003-315988). There has been disclosed such a technique that in aninorganic resist material made of an incomplete oxide of a transitionmetal disclosed in Patent Document 2, even by the exposure using avisible laser of about 405 nm, the exposure of patterns smaller than thespot diameter can be performed showing to the thermal recordingcharacteristics. An attention has been paid to such a technique as atechnique that is useful to a mastering technique of the optical diskcorresponding to the BD format of a ROM (Read Only Memory) or therecording density higher than that in the BD format.

The incomplete oxide of the transition metal disclosed here denotes acompound in which an oxygen content is deviated in such a direction asto be smaller than that of a stoichiometric composition according to avalence number which the transition metal can have, that is, a compoundin which the content of oxygen in the incomplete oxide of the transitionmetal is smaller than the oxygen content of the stoichiometriccomposition according to the valence number which the transition metalcan have. In the incomplete oxide of the transition metal, since alatent image forming portion by the exposure has been oxide-altered, itis soluble to an alkali developer and microfabrication of the master forthe optical disk can be realized.

The exposure beam diameter d in the case of the laser wavelength of 406nm is obtained byd=1.22×406 [nm]/0.85=about 583 [nm]

The following relation is obtained.Exposure beam diameter (583 nm)>>track pitch (320 nm) in the BD formatof the ROM

As mentioned above, by using the inorganic resist, the optical diskformat of the track pitch which is equal to about the half of theexposure beam diameter can be formed.

A technique in which by using a phase-change (inorganic) resist and alaser of the wavelength of 480 nm, super-high density concave/convexmarks each having a diameter of about 40 nm as shown in an SEM (ScanningElectron Microscope) photograph in FIG. 4 can be formed has beendisclosed in Non-Patent Document 1 (Nano-pattern machining technique ofa phase-change recording film for a disk, “HAITAKKU”, Hitachi, Ltd.,pages 9-10, April, 2004).

SUMMARY OF THE INVENTION

However, to form the super-high density optical disk format, it isnecessary to form a format in which a fluctuation (that is, deviation)in the groove width is small, a variation of the track pitch is small,and a height change of the land portion is small. In Non-Patent Document1 mentioned above, the super-high density ROM in which each of a pitlength and a space length are equal to 40 nm has been made on anexperimental basis by using the phase-change (inorganic) resist and thelaser of the wavelength of 480 nm. However, as will be understood fromFIG. 4, since the widths and lengths of the pits are not uniform and thevariation in the track pitch is large, such a disk does not satisfy thesufficient characteristics as an optical disk format. The modulationsuch as 1-7 modulation or the like is not executed and the ROM format ofthe optical disk is not realized.

Even if the disk is directly exposed by a pulse signal of the 1-7modulation by using the inorganic resist, there is such a problem thatthe pit width is changed in dependence on the pit length, a heat isaccumulated in a rear portion of the exposure beam, there is a change inwidth in one pit, or the like.

As mentioned above, if the ROM format of the super-high density opticaldisk whose track pitch is equal to or less than 300 nm is formed, thismeans that the fluctuation in the pit width due to the pit length issmall, the width change in one pit is small, and the track pitchvariation is also small. It is, therefore, very difficult to realizesuch a format by the related art.

Upon manufacturing of another device such as a semiconductor or the likewithout limiting to the optical disk, it is also difficult to form theaccurate and extremely fine concave and convex portions in which theheight change is small.

It is, therefore, desirable to provide a recording method, a master foran optical disk, and an optical recording medium which are suitable whenthey are applied to a high-density optical disk whose track pitch isequal to or less than 300 nm and in which recording and reproducingcharacteristics such as tracking servo characteristics and the like areexcellent and the high density can be practically realized.

According to an embodiment of the present invention, there is provided arecording method whereby an inorganic resist made of an incomplete oxideof a transition metal is formed as a film onto a substrate and a latentimage corresponding to pits is formed onto the inorganic resist byexposure,

wherein the exposure is performed by a laser beam whose intensity hasbeen modulated by a pulse signal whose pulse height decreases in a rearportion in a length direction of the pit, thereby forming a format of atrack pitch smaller than a recording beam diameter (track pitch/exposurebeam diameter=0.333 to 0.833).

According to another embodiment of the present invention, there isprovided a recording method whereby an inorganic resist made of anincomplete oxide of a transition metal is formed as a film onto asubstrate and a latent image corresponding to pits is formed onto theinorganic resist by exposure, comprising the steps of:

feedback controlling an intensity of a laser beam so that a changeamount of an exposure amount is equal to or less than ±1.0%;

performing the exposure by the laser beam whose intensity has beenmodulated by a pulse signal whose pulse height decreases in a rearportion in a length direction of the pit; and

forming a format of a track pitch smaller than a recording beam diameter(track pitch/exposure beam diameter=0.333 to 0.833) by using ahigh-precision track feeding servo whose track pitch variation is equalto or less than ±3 nm.

According to still another embodiment of the present invention, there isprovided a master for an optical disk having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a width change of a pit having a maximum length among a plurality of pitlengths is equal to or less than 80.3%.

According to still another embodiment of the present invention, there isprovided a master for an optical disk having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a fluctuation of a pit width due to a pit length is equal to or lessthan 89.3%.

According to still another embodiment of the present invention, there isprovided a master for an optical disk having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a track pitch variation is equal to or less than ±3 nm.

According to still another embodiment of the present invention, there isprovided an optical recording medium having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a width change of a pit having a maximum length among a plurality of pitlengths is equal to or less than 80.3%.

According to still another embodiment of the present invention, there isprovided an optical recording medium having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a fluctuation of a pit width due to a pit length is equal to or lessthan 89.3%.

According to still another embodiment of the present invention, there isprovided an optical recording medium having a format in which a trackpitch is equal to 300 to 120 nm, wherein

a track pitch variation is equal to or less than ±3 nm.

According to an embodiment of the present invention, the super-highdensity optical recording medium whose track pitch is equal to or lessthan 300 nm can be realized.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of an optical recordingapparatus to which an embodiment of the invention can be applied;

FIGS. 2A to 2E are schematic diagrams for explaining a pulse strategy inthe embodiment of the invention;

FIGS. 3A to 3C are brightness diagrams of an AFM showing pit shapes indifferent formats;

FIG. 4 is a brightness diagram showing a result when concave/convexmarks are observed by an SEM;

FIG. 5 is Table 1 showing an exposure wavelength of a laser, a trackpitch, a wobble amount, and an exposure beam diameter in a BD format;

FIG. 6 is Table 2 showing a voltage setting of a pulse strategy A;

FIG. 7 is Table 3 showing a voltage setting of a pulse strategy B;

FIG. 8 is Table 4 showing a voltage setting of a pulse strategy C;

FIG. 9 is Table 5 showing measurement results of pit shapes regarding anevaluating disc I; and

FIG. 10 is Table 6 showing measurement results of pit shapes regardingan evaluating disc II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention will be described herein below withreference to the drawings. The embodiment provides a method of forming aproper pit shape in a super-high density optical disk medium of a ROM.FIG. 1 shows a high-precision laser exposing apparatus in the embodimentof the invention.

In the embodiment, the high-precision exposure is enabled mainly by thefollowing component elements.

-   -   1) A laser light source of quartic harmonics of a YAG (Yttrium        Aluminum Garnet) laser having a wavelength of 266 nm and an        objective lens having a numerical aperture NA=0.9 are used and        an exposure beam is reduced.    -   2) High-precision light output control is made and a change        amount of an exposure amount is set to a value within ±1.0%.        Further, by performing exposure by a pulse strategy whose pulse        height decreases in a rear portion, a fluctuation of a pit width        due to a pit length is reduced and a width change in one pit is        reduced.    -   3) A laser scale of super-high resolution (0.28 nm) is used for        a high-precision track feeding servo, a slide motor drive of a        linear amplifier is used (high gain of about tens of kHz), and a        track pitch variation is set to a value within ±3 nm.

An inorganic resist of super-high resolution is used as a resistmaterial, thereby realizing a super-high density optical disk format inwhich a fluctuation of a groove width is small, a track pitch variationis also small, a height change of a land portion is small, and a trackpitch is equal to or less than 300 nm.

A resist material which is used in the embodiment is an incomplete oxideof a transition metal. It is now defined that the incomplete oxide ofthe transition metal is a compound in which an oxygen content isdeviated in such a direction as to be smaller than that of astoichiometric composition according to a valence number which thetransition metal can have, that is, a compound in which the content ofoxygen in the incomplete oxide of the transition metal is smaller thanthe oxygen content of the stoichiometric composition according to thevalence number which the transition metal can have.

For example, the case where an oxide shown by a chemical formula MoO₃ isused as an oxide of the transition metal will be described as anexample. When an oxide state of the chemical formula MoO₃ is convertedinto a composition ratio Mo_(1-x)O_(x), it will be understood that theoxide in the case where x=0.75 is a complete oxide and the oxide in thecase where 0<x<0.75 is the incomplete oxide whose oxygen content lacksmore than that of the stoichiometric composition.

In the transition metals, there are metals in which one element can formthe oxide of a different valence number. In such a case, it is assumedthat the case where the actual oxygen content lacks more than that ofthe stoichiometric composition according to the valence number which thetransition metal can have lies within the range in the embodiment. Forexample, as for Mo, the tervalent oxide (MoO₃) mentioned above is moststable. Another univalent oxide (MoO) also exists. In this case, when itis converted into the composition ratio Mo_(1-x)O_(x), it will beunderstood that the oxide in the case where 0<x<0.5 is the incompleteoxide whose oxygen content lacks more than that of the stoichiometriccomposition. The valence number of the transition metal oxide can beanalyzed by a commercially available analyzer.

Such an incomplete oxide of the transition metal exhibits absorbingcharacteristics to the ultraviolet rays or the visible light and byirradiating the ultraviolet rays or the visible light to the incompleteoxide, its chemical nature is changed. Thus, as will be explained indetail hereinafter, although it is the inorganic resist, a differencebetween an etching speed of an exposing portion and that in anon-exposing portion occurs in the developing step, that is, what iscalled a selection ratio is obtained. In the resist material made of theincomplete oxide of the transition metal, since a film particle size issmall, the patterns in a boundary portion between the non-exposingportion and the exposing portion become clear and resolution can beraised.

In the incomplete oxide of the transition metal, since thecharacteristics as a resist material are changed in dependence on adegree of oxidization, a proper optimum oxidization degree is selected.For example, in the incomplete oxide whose oxygen content is fairlysmaller than that of a stoichiometric composition of a complete oxide ofthe transition metal, there is such an inconvenience that a largeirradiating power is necessary in the exposing step, it takes a longtime for the developing process, and the like. Therefore, it ispreferable to use the incomplete oxide whose oxygen content is slightlysmaller than that of the stoichiometric composition of the completeoxide of the transition metal.

As specific transition metals forming the resist material, Ti, V, Cr,Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, Ag, and the like can bementioned. Among them, it is preferable to use Mo, W, Cr, Fe, and Nb. Itis particularly preferable to use Mo and W from a viewpoint that a largechemical change is obtained by the ultraviolet rays or the visiblelight.

As incomplete oxides of the transition metal, besides the incompleteoxide of the transition metal of one kind, all of an incomplete oxideadded with the second transition metal, an incomplete oxide furtheradded with a plurality of kinds of transition metals, an incompleteoxide added with an element other than the transition metal, and thelike are incorporated in the scope of the embodiment. Particularly, anincomplete oxide containing a plurality of kinds of metal elements ispreferable. Besides the incomplete oxide of the transition metal of onekind, in the cases of the incomplete oxide added with the secondtransition metal and the incomplete oxide further added with three ormore of kinds of transition metals, it is considered that a part of thetransition metal atoms of one kind having a crystalline structure havebeen replaced by other transition metal atoms. However, whether or notthe oxide is the incomplete oxide is discriminated by checking whetheror not the oxygen content lacks more than that of the stoichiometriccompositions which those plurality of kinds of transition metals canhave.

As an element other than the transition metal, at least one kind of Al,C, B, Si, Ge, and the like can be used. By using a combination of two ormore kinds of transition metals or by adding the element other than thetransition metal, a crystal grain of the incomplete oxide of thetransition metal decreases. Therefore, the boundary portion between theexposing portion and the non-exposing portion becomes further clear andthe resolution is further raised. Exposing sensitivity can be improved.

The foregoing resist material may be manufactured by a sputtering methodin an (Ar+O₂) atmosphere using a target containing a predeterminedtransition metal. For example, a ratio of O₂ to the whole flow amount ofintroducing gases into a chamber is set to 5 to 20% and a gas pressureis set to a gas pressure (1 to 10 Pa) in the ordinary sputtering.

In the embodiment, the resist layer is formed as follows. First, a filmof the resist layer made of the incomplete oxide of the transition metalis formed on the substrate whose surface has sufficiently been smoothed.As a specific film forming method, for example, a method of forming thefilm by the sputtering method in the argon and oxygen atmosphere byusing a sputtering target made of a simple substance of the transitionmetal can be mentioned. In this case, the oxidization degree of theincomplete oxide of the transition metal can be controlled by changingconcentration of an oxygen gas in a vacuum atmosphere. In the case offorming the film of the incomplete oxide of the transition metalcontaining two or more kinds of transition metals by the sputteringmethod, a plurality of kinds of transition metals are mixed by alwaysrotating the substrate over different kinds of sputtering targets. Amixture ratio is controlled by changing a sputtering applying power ofeach target.

Besides the sputtering method in the oxygen atmosphere using the metaltargets mentioned above, by preliminarily executing the sputtering inthe normal argon atmosphere by using the target made of the incompleteoxide of the transition metal containing a desired amount of oxygen, afilm of the resist layer made of the incomplete oxide of the transitionmetal can be also similarly formed.

Further, besides the sputtering method, a film of the resist layer madeof the incomplete oxide of the transition metal can be also easilyformed by an evaporation depositing method.

As a material of the substrate, silicon, glass, plastics such aspolycarbonate or the like, alumina titanium carbide, nickel, or the likecan be used.

A film thickness of resist layer can be arbitrarily set. For example, itmay be set to a thickness within a range from 10 to 80 nm.

The resist layer formed on the substrate is exposed by an opticalrecording apparatus. The optical recording apparatus used in theembodiment will now be described with reference to FIG. 1. A resistlayer 11 obtained by forming the inorganic resist film onto a master 10made of silicon or the like as mentioned above is exposed, so that alatent image of pits is formed on the resist layer 11.

When the latent image is formed on the resist layer 11, the master 10 isattached onto a turntable 12 provided on a mobile optical table 20. Whenthe resist layer 11 is exposed, the master 10 is rotated by theturntable 12 and the master 10 is moved in parallel by the mobileoptical table 20 so that the exposure of desired patterns is executed onthe whole surface of the resist layer 11. The master 10 is rotated, forexample, at a constant linear velocity.

In such a laser cutting apparatus, the resist layer 11 on the master 10is exposed so that tracks of a desired track pitch, for example, 160 nmare formed. A rotational speed of the turntable 12 is controlled by aspindle servo 13 so that a linear velocity in the longitudinal directionof the tracks is equal to, for example, 2.00 [m/sec]. A feed pitch ofthe mobile optical table 20 is controlled by controlling the operationof an air slider 32 by a high-precision track feeding servo 33. Thefeeding servo 33 makes positioning control by using a laser scale 31having a wavelength of, for example, 0.78 [μm] and resolution of, forexample, 0.28 nm as a reference. By such control, a latent image of agroove pattern in a data recording area can be formed onto the resistlayer 11 on the master 10 at the track pitch of 160 nm.

The slide motor is driven by the linear amplifier of a wide band up toabout tens of kHz by using the laser scale 31 having the super-highresolution (0.28 nm) and the operation of the air slider 32 iscontrolled by the slide motor at a high gain. Thus, a latent image of apit pattern in which a track pitch variation is equal to or less than ±3nm can be formed on the resist layer 11.

A laser beam for recording is emitted from a laser light source 14. Anarbitrary light source can be used as a light source 14. It ispreferable to use a light source which emits a laser beam of a shortwavelength, for example, a deep ultraviolet laser such as so-called“DeepUV” whose wavelength is equal to a value in the 200-nm range.Specifically speaking, for example, a laser source which oscillates therecording laser beam of quartic harmonics (λ=266 nm) of a YAG laser isused.

The laser beam emitted from the light source 14 goes straight as aparallel beam, enters a laser intensity modulator such as an EOM(Electro Optic Modulator) 15 as a modulator using an electro-opticaleffect, and its intensity is modulated by the EOM 15. The laser beamwhose intensity has been modulated by the EOM 15 enters a beam splitterBS1 for separating the optical path through an analyzer. A part of thelaser beam transmitted through the beam splitter BS1 enters aphotodetector (PD) 16 arranged on the transmission optical path.

A detection signal of the photodetector 16 is supplied to an APC (AutoPower Controller) 17. Feedback control is made to the EOM 15 by the APC17. A light output power of the laser beam transmitted through the EOM15 is changed by the APC 17 in correspondence to the linear velocity andthe recording can be executed while an exposure amount per unit area isheld constant. For example, a change amount of the exposure amount canbe suppressed within ±1.0%.

The laser beam reflected by the beam splitter BS1 is guided to amodulation optical system OM. Two lenses 21 and 23 and one AOM (AcousticOptical Modulator) 22 are arranged for the modulation optical system OM.The lenses 21 and 23 and the AOM 22 are arranged so that the laser beamwhich has entered as a parallel beam and a lattice plane satisfy a Braggcondition. A synthetic quartz is suitable as an acoustic optical elementwhich is used for the AOM 22.

A predetermined signal is supplied to the AOM 22 from a driving circuit24. This signal is, for example, a 1-7 modulated pulse signal. Forexample, in the “Blu-ray Disc (registered trademark of SonyCorporation)” format, a system called “1-7PP (Parity Preserve/Prohibit)RMTR” is used as an encoding system. This encoding system is a system inwhich one or more number of “0” are included between “1” and “1” and twobits are replaced by three bits. As will be explained hereinafter, inthe case of forming a pit of a length of nT (n is a positive integer),the above predetermined signal is (n−1) pulse signals, and each pulsehas a large amplitude at the head and has a small amplitude in the rearportion.

The AOM 22 uses a principle in which an intensity of a primarydiffracted light in Bragg diffraction is almost proportional to anultrasonic power. The AOM 22 modulates the ultrasonic power on the basisof the modulation signal and the intensity of the laser beam islight-modulated.

To realize the Bragg diffraction, a positional relation and a positionof the AOM 22 to an optical axis of the laser beam are set so as tosatisfy the following Bragg condition.2d sin θ=nλwhere,

-   -   d: lattice interval    -   λ: wavelength of the laser beam    -   θ: angle between the laser beam and the lattice plane    -   n: integer

The modulation light outputted from the modulation optical system OM isreflected by a mirror M2, horizontally guided over the mobile opticaltable 20, and enters a beam expander (BEX) 26. The laser beam whose beamdiameter has been increased by the beam expander 26 is reflected by amirror M3 and irradiated onto the resist layer 11 on the master 10through an objective lens 27. By increasing the beam diameter, anumerical aperture NA of the objective lens 27 can be set to 0.9. Thelatent image of the pits is formed in the data recording area of thetrack on the resist layer 11.

Subsequently, a developing process is executed to the resist layer 11 onthe master 10. For example, the resist layer 11 coated on the master 10is a positive type resist and a portion where the latent image has beenformed by the resist light is melted by the development. For example,this portion corresponds to the pit and the portion remaining after thedevelopment corresponds to the land. In more detail, the non-developedmaster 10 is put onto the turntable of a developing apparatus (notshown). While the master 10 is rotated together with the turntable 12, adeveloper is dropped onto the surface of the master 10 and the resistlayer 11 on this surface is developed. Thus, the resist master 10 onwhich the pits have been patterned can be obtained.

Subsequently, an electroconductive film layer as a nickel coating filmis formed onto the concave/convex patterns of the optical disk master byan electroless plating method or the like. The optical disk masterformed with the electroconductive film layer is attached to anelectroforming apparatus. A nickel plating layer is formed onto theelectroconductive film layer by an electroplating method so as to have athickness of about 300±5 [μm]. Subsequently, the nickel plating layer ispeeled off from the master with the nickel plating layer by using acutter or the like. The resist on the signal forming surface of thenickel plating layer is cleaned by using acetone or the like, therebyforming a stamper. For example, a master stamper is formed from themaster 10 and a mother stamper whose concave/convex patterns arereversed, that is, are opposite to those of the master stamper isfurther formed.

The mother stamper is attached to a die of an injection moldingapparatus and a resin such as polycarbonate (refractive index: 1.59) orthe like is injected into a cavity, so that a disc substrate to whichthe concave/convex patterns of the stamper have been transferred isformed. At this time, the resin which is used for the disc substrate hasbeen plasticized by the heat so that it can be filled into the die at ahigh speed. The injection molded disc substrate having a thickness of1.1 mm is cooled to 30° C. or lower. After that, a thin metal film ofaluminum alloy, silver, or the like is formed on the pit surface side byusing a sputtering apparatus, so that a reflecting film is formed.

Subsequently, an ultraviolet hardening resin is dropped as an adhesiveagent onto the disc substrate on which the reflecting layer film hasbeen formed and the disc substrate is uniformly coated with the resin bya spin coating method. After that, the coating surface of theultraviolet hardening resin on the disc substrate and the polycarbonatefilm are held at the opposite positions and subsequently adhered. Theadhering process of the polycarbonate film is executed in the vacuum.This is because it is necessary to prevent that wrinkles and gaps occurin the adhering surface of the disc substrate and the polycarbonate filmand a reading error occurs.

Subsequently, by irradiating ultraviolet rays onto the disc to which thepolycarbonate film has been adhered, the ultraviolet hardening resin ishardened, thereby adhering the disc substrate and the polycarbonatefilm. Further, a hard coating agent of an ultraviolet hardening type isdropped onto the polycarbonate film adhered to the disc and thepolycarbonate film is uniformly coated with the coating agent by thespin coating method. After that, the hard coating agent is hardened byirradiating the ultraviolet rays again, thereby forming a hard coatinglayer. Thus, the disc is completed. A disc for evaluating is alsosimilarly formed.

In the encoding system called “1-7PP (Parity Preserve/Prohibit) RMTR” asmentioned above, pits of eight kinds of lengths of (2T, 3T, 4T, 5T, 6T,7T, 8T) and 9T existing in a part of the recording area as pit lengthsare formed. The signal in a recording signal portion becomes almostrandom signal patterns of 2T to 9T. A single signal pattern of arelatively small pit such as 2T, 3T, or the like is formed in adedicated monitor signal portion.

In the laser exposing apparatus in the related art, the intensity of thelaser beam has been modulated by a pulse waveform of a 1-7 modulationsignal shown in FIG. 2A by using the AOM. FIG. 2A shows the pulsewaveform in which (a 2T mark, a 2T space, an 8T mark, a 3T space, a 3Tmark, a 2T space, and a 4T mark) continue.

Even if the laser beam for exposing is directly modulated by the 1-7modulation signal as mentioned above, as shown in FIG. 2B, in the formedevaluating disc, there are such problems that the pit width is changeddue to the pit length, the heat is accumulated in the rear portion ofthe exposure beam, a change in width in one pit occurs, and afluctuation in pit width depending on the pit length occurs. Theinvention can be also applied to encoding systems other than the 1-7modulation.

An embodiment of the invention intends to solve such a problem. In anembodiment of the invention, it is assumed that the laser beam forexposing is not continuously set to the high level for a period of timecorresponding to the width of mark but is set to the pulse signals of apredetermined period and the level of the pulse signals is set to belarger as the pulse approaches the head pulse. A setting method of thenumber of pulse signals for modulating and their levels is called “pulsestrategy”. Assuming that a mark length is equal to nT, the number ofpulses is equal to (n−1). In the example shown in FIGS. 2A to 2E, thenumbers of pulses corresponding to the marks of 2T, 8T, 3T, and 4T areset to 1, 7, 2, and 3, respectively. In this instance, three kinds ofpulse strategies A, B, and C are set and evaluation is made with respectto each pulse strategy.

As shown in FIG. 2C, three kinds of levels of V1, V2, and V3 (V1>V2>V3)exist as levels of the pulses. The method whereby the maximum level V1is used only for the mark of 2T corresponds to the pulse strategies Aand B. A voltage setting of the pulse strategy A is shown in Table 2 ofFIG. 6. In the pulse strategy A, V1=1000 mV, V2=950 mV, and V3=900 mV.In the case of forming the pits corresponding to the marks of 3T to 6T(referred to 3T pit to 6T pit), the level of the head pulse is set to V2and the levels of the remaining (n−2) pulses are set to V3. In the caseof forming the pits corresponding to the marks of 7T and 8T (referred to7T pit and 8T pit), the levels of the pulses are equally set to V2.

A voltage setting of the pulse strategy B is shown in Table 3 of FIG. 7.In the pulse strategy B, V1=1000 mV, V2=900 mV, and V3=800 mV. In thecase of forming the 3T to 8T pits, the level of the head pulse is set toV2 and the levels of the remaining (n−2) pulses are set to V3.

As shown in FIG. 2D, four kinds of levels of V11, V12, V13, and V14(V11>V12>V13>V14) exist as levels of the pulses. The method whereby themaximum level V11 is used only to form the 2T pit corresponds to thepulse strategy C. A voltage setting of the pulse strategy C is shown inTable 4 of FIG. 8. In the pulse strategy C, V11=1000 mV, V12=900 mV,V13=800 mV, and V14=700 mV. In the case of forming the 3T to 8T pits,the level of the head pulse is set to V12, the level of the second pulseis set to V13, and the levels of the remaining (n−3) pulses are set toV14.

As will be explained hereinafter, by using the pulse strategies B and C,as shown in FIG. 2E, the pit in which the fluctuation of the pit widthis small can be formed without causing such problems that the pit widthis changed due to the pit length, the heat is accumulated in the rearportion of the exposure beam, the change in width in one pit occurs, thefluctuation in pit width depending on the pit length occurs, and thelike.

An evaluating disc I is formed by using each of the foregoing pulsestrategies A, B, and C and evaluation is made by using an evaluatingapparatus (reproducing apparatus) having a hyper-hemispherical ball lensoptical pickup (wavelength 405 nm, NA=2.1). In the evaluating disc I, atrack pitch is equal to 160 nm and a length of 2T on the disc is equalto 75 nm. Nine areas on the disc are set. The pulse strategy A, pulsestrategy B, and pulse strategy C are applied to the regions each ofwhich is formed by three areas in order from the inner rim side,respectively. Areas 1 to 9 are obtained by dividing a range from 20 mmto 28 mm as a radius of the disc on a unit basis of 1 mm. With respectto each pulse strategy, an emitting power of the laser beam is changedto three kinds of powers (2.16 mW, 2.30 mW, 2.44 mW). With respect tothe evaluating disc I, measurement results of the pit shapes are shownin Table 5 of FIG. 9.

FIGS. 3A to 3C show brightness diagrams of an AFM (Atomic ForceMicroscope) of the pit shapes of the evaluating disc I. In FIGS. 3A to3C, the brightness diagrams of the areas 1, 2, and 3 using the pulsestrategy A are the three diagrams shown at the upper stage. Thebrightness diagrams of the areas 4, 5, and 6 using the pulse strategy Bare the three diagrams shown at the middle stage. The brightnessdiagrams of the areas 7, 8, and 9 using the pulse strategy C are thethree diagrams shown at the lower stage. The brightness corresponds to adepth of pit. In FIGS. 3A to 3C, the pit is shown as a dark portion,that is, a concave portion. In the formed evaluating disc, the pit is aconvex portion. However, since FIGS. 3A to 3C are diagrams showing thestate at the stage of the stamper, the concave/convex portions are shownin a reversed state.

The 8T width as one of evaluating items indicates the minimum value andthe maximum value of the width of pit in the case of forming the 8T pit.The 8T width fluctuation (that is, deviation) indicates a range of thefluctuation of the 8T width obtained by dividing the minimum value bythe maximum value. It means that the more the 8T fluctuation is close to100%, the smaller the width fluctuation is. The 2T width indicates awidth of pit in the case of forming the 2T pit. The (2T/8T) widthfluctuation indicates a ratio between the average value of the 2T widthand that of the 8T width. It means that the more the (2T/8T) widthfluctuation is close to 100%, the smaller the width fluctuation is inthe case where the pits of different lengths have been formed. In thecase where the 8T pit is formed, the fluctuation of the pit width islargest as compared with those in the case of forming the pits of otherlengths. Therefore, the 8T mark is used as an item of the evaluation.

As shown in Table 5 mentioned above, in the pulse strategy A, the widthfluctuation of the 8T pit is larger and the ratio of the (2T/8T) widthfluctuation is smaller than those in the other pulse strategies B and C.In all of the pulse strategies, if the power of the laser beam isinsufficient, the fluctuation of the pit width increases. However, thepower of the laser beam can be properly set. Further, it will be alsounderstood from the brightness diagrams of the AFM of FIGS. 3A to 3Cthat the fluctuation of the pit width is large in the pulse strategy A.

In the evaluating disc I, the stable tracking servo and the stable pitreproduction can be performed, in the areas 5, 6, 8, and 9, in otherwords, the reproduction in which the error rate is sufficiently smallcan be performed. In those areas, since the fluctuation of the pit widthdue to the pit length is small and the fluctuation of the width in onepit is also small, the stable reproduction can be performed.

Another evaluating disc II is formed. In the evaluating disc II, a trackpitch is changed to 300 nm, 200 nm, and 120 nm and a length of 2T pit ischanged to 141 nm, 94 nm, and 56 nm by using the pulse strategies B andC, respectively. The areas 1 to 9 are obtained by dividing the rangefrom 20 mm to 28 mm as a radius of the disc on a unit basis of 1 mm.Different parameters are used in the areas 1 to 9. With respect to theevaluating disc II, measurement results of the pit shapes are shown inTable 6 of FIG. 10.

It will be understood from the measurement results of Table 6 that thestable tracking servo can be performed and the pit can be stablyreproduced in the areas 2, 3, 5, 6, 8, and 9.

With respect to the evaluating discs I and II mentioned above,conditions in which the stable tracking servo can be performed and thepit can be stably reproduced are as follows.

-   -   (1) The width fluctuation of one pit (for example, 8T pit) (8T        width fluctuation) is equal to or less than 80.3%    -   (2) The fluctuation of the pit width due to the pit length is        equal to or less than 89.3%.

Further, when an attention is paid to the high-precision format in whichthe track pitch variation is equal to or less than ±3 nm, in the formatsof the areas 2, 3, 5, 6, 8, and 9 (track pitch is equal to 300 to 120nm), it is necessary that (the track pitch variation)/(the track pitch)is equal to or less than ±0.03 nm.

As mentioned above, in the embodiment of the invention, there are used:the laser light source 14 having the wavelength of 266 nm; the objectivelens 27 having the numerical aperture NA=0.9; the high-precision lightoutput control in which the change amount of the exposure amount isequal to or less than ±1.0%; the proper pulse strategies B and C; andthe high-precision track feeding servo whose track pitch variation isequal to or less than ±3 nm. Therefore, the ROM type optical disk formatwhose track pitch is equal to or less than 300 nm can be realized.

The laser having the wavelength of 266 nm is converged by the objectivelens having the NA of 0.90 and the format of the track pitch (300 nm to120 nm) which is extremely smaller than the exposure beam diameter (360nm) can be realized.(The track pitch)/(the exposure beam diameter)=0.333 to 0.833

Although the embodiment of the invention has specifically been describedabove, the invention is not limited to the foregoing embodiment butvarious modifications based on the technical idea of the invention arepossible. For example, the numerical values mentioned in the aboveembodiment are merely shown as an example and other different numericalvalues may be used as necessary. The cutting apparatus may have anyconstruction other than that shown in FIG. 1. Further, the opticalrecording medium is not limited to the disc shape but may have a cardshape.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A recording method whereby an inorganic resist made of an incompleteoxide of a transition metal is formed as a film onto a substrate and alatent image corresponding to pits is formed onto the inorganic resistby exposure, comprising the steps of: feedback controlling an intensityof a laser beam so that a variation in exposure amount is equal to orless than ±1.0%; performing the exposure by the laser beam whoseintensity has been modulated by a pulse signal whose pulse height isfirst asserted at first maximum height and then asserted one or moretimes at intermediate non-zero heights lower than the first maximumheight in a rear portion in a length direction of the pit; and forming atrack pitch smaller than a recording beam diameter by using ahigh-precision track feeding servo whose track pitch variation is equalto or less than ±3 nm.
 2. The method according to claim 1, wherein aratio of track pitch/exposure beam diameter is in the range of 0.333 to0.833.